Electrolytic cell for the production of aluminum



June 21, 1966 J. HENRY ETAL ELECTROLYTIC CELL FOR THE PRODUCTION OFALUMINUM Filed June 11, 1962 5 Sheets-Sheet 1 CARBON FIG. El

CURRENT FIELD FLEX 3O ANODE 24 ELECTROLYTE 22 METAL LAYER I4 CELL LININGANODES FIEl INSULATION 40 RHM BARS 4 FIG. Ed

ANODES gi jg CURRENT 46 FIELD RHM BARS F I E. E In FIG. El

INVENTOR. ROBIN D. HOLLIDAY JACK L. HENRY June 21, 1966 J. L. HENRY ETAL3,257,307

ELECTROLYTIC CELL FOR THE PRODUCTION OF ALUMINUM Filed June 11, 1962 5Sheets-Sheet 2 CELL WALL cAPPED RHM BARS PIE. 7

INVENTOR. ROBIN D. HOLLIDAY JACK L. HENRY June 21, 1966 J. L. HENRY ETAL3,

ELECTROLYTIC CELL FOR THE PRODUCTION OF ALUMINUM Filed June 11, 1962 5Sheets-Sheet 5 I04 ELECTROLYTE METAL PAD LINING INSULATION INVEN R.ROBIN D HOLLID JACK L. HENRY United States Patent 3,257,307 ELECTROLYTICCELL FOR THE PRGDUCTION 0F ALUMINUM Jack L. Henry, Los Altos, and RobinD. Holliday, San

Jose, Calif., assignors to Kaiser Aluminum & Chemical Corporation,Oakland, alif., a corporation of Delaware Filed June 11, 1962, Ser. No.201,669 8 Claims. (Cl. 204231) The present invention relates to a novelelectrolytic cell arrangement. More particularly, the invention relatesto an improved cell design utilizing refractory hard metalcurrent-conducting elements of the type recently proposed for use inelectrolytic cells for the production of aluminum. The inventioncontemplates cell designs wherein the refractory hard metalcurrentconducting elements are protected from corrosive action duringoperation and result in increased life of the current-conductingelements.

Recent innovations in the design of electrolytic cells for theproduction of aluminum incorporate the use of refractory hard metalcurrent-conducting elements in the cathodic system in preference topreviously used carbon material. Electrolytic cells utilizing refractoryhard metal current-oonducting elements provide a number of advantages inoperation which improve the efficiency and lower the cost of producingaluminum. The present invention provides for additional savings in theuse of refractory hard metal current-conducting elements by prolongingthe life during operation in the cell of the elements and reducingdeterioration by corrosion. Refractory hard metal materials, asrecognized in the art, include nonmetallic materials such as thecarbides and borides of titanium, zirconium, tantalum and niobium, andmixtures thereof.

Among the electrolytic cell designs utilizing refractory hard metalelements are the so-called top entering designs wherein the refractoryhard metal bars .are extended through the electrolyte into the metalliccathode layer in the cell from the top where they. are supported orsuspended from overlying superstru-ctnres. One principal cause ofdeterioration of top entering refractory hard metal currentconductingelements during operation in an electrolytic cell is corrosion attack inthe crust zone. The crust zone in the electrolytic cell is the top layerwhich overlies the electrolyte surface. The crust is gener-allycomprised of a mixture of alumina, aluminum fluoride, cryolite and othercell addition materials. The crust forms a hardened outer surface whichperforms certain useful functions, e.g., heat insulation. Top enteringrefractory hard metal current-conducting elements extend through thecrust layer into the electrolyte. Deterioration of the elements bycorrosion attack in the crust zone comes about by electrolyticconduction through the crust between the carbon anodes and therefractory hard metal cathodic current-conducting elements. Theelectrolytic conduction results in deposition of elemental sodium at thesurface of the refractory hard metal element. Oxidation and hydrolysisof the sodium then results in a highly corrosive mixture of NaO'I-I, NaO, and other corrosive materials.

As an illustration of this corrosive attack on the refractory hard metalelements, the following experiments are described to show thatsufficient current may be passed through solid flux (constituting thecrust) to allow electrolysis and corrosion to occur.

Example I A small carbon-lined clay crucible filled with cryolite,Temperature, C. Applied Voltage Current, amps.

An alkaline reaction and strong sodium flame are obtained from thecathode graphite after removal indicating the presence of sodiumdeposition at the cathode. The anode was neutral with very little sodiumflame coloration.

Example II In another experiment a in. titanium boride bar was used as acathode. The electrolyte was a mixture of cryolite with approximately 5%CaF and 5% A1 0 to more resemble the composition of the crust. The anodeand cathode bars were rigidly gripped and held parallel. The temperaturewas controlled at 700 C. and the applied voltage maintained at 6.0 voltsand with a current of 3.7 amps. After 70 hours it was found uponinspection that the graphite anode had been oxidized away. The flux wasbroken away from the cathode bar which was found to show the beginningof deterioration with necking about A in. below the electrolyte surface.Light sand blasting showed the attack to be most pronounced on the sidefacing the anode rod. In addition, the flux around the neck exhibited astrongly alkaline reaction whereas the area around the anode did not.

According to the invention there is provided a method of preventingcorrosion attack by interposing current-conducting elements in the crustbetween the carbon anodes of the cell and the refractory hard metalcurrent-conducting bars. These elements can be connected to the cathodebus bar system and cathodic deposition will be induced on theintervening elements rather than on the refractory hard metal bars. Incorrison terminology the intervening elements can be described assacrificial cathodes.

The invention is further described and explained by reference to theaccompanying drawings wherein:

FIG. 1 is a schematic view partly in section depicting the generalarrangement of the sacrificial cathode with respect to the refractoryhard metal current-conducting element and the normal anode system of theelectrolytic cell.

FIGS. 2a and 2b depict the anode-cathode arrangement, illustrating thecurrent field and arrangements without and with the interveningsacrificial cathodes, respectively.

FIGS. 3, 4, and 5 describe some simple forms of sacrificial electrodeswhich may be employed according to the invention.

FIG. 6 is a top schematic view of another arrangement utilizing aprotective sacrificial cathode barrier for the refractory hard metalcathodic elements.

FIG. 7 is a schematic side view of the arrangement in 'FIG. 6illustrating one way the sacrificial cathode barrier may be connected toa cathodic system.

FIG. 8 is an illustration of another arrangement of sacrificial cathodeand refractory hard metal cathode displacement wherein the refractoryhard metal elements are centrally disposed with anodes and sacrificialcathode assemblies on both sides of the refractory hard metal cathodes.

not a highly critical factor.

FIG. 9 is a schematic side view of the plan illustrated in FIG. 8.

In FIG.'1 the relationship between the sacrificial cathode 30 and therefractory hard metal cathodic currentc-onducting element is illustratedpartially in section Within an electrolytic cell 10. As can be seen theelectrolytic cell It comprises a refractory side wall 12 which, togetherwith bottom cell lining l4 and insulation 16, form the cell containerfor metal pad 22, molten electroly'te24, and crust layer 26. Crust layer26 is the top layer and covers the electrolyte. sacrificial cathode 30is electrically connected by electrical connection means 32 to therefractory hard metal cathode at the cap member 1.8 of the cathodeelement 20. The electrical connection means 32 may be suitably securedby fastening means 34- to the cap 18.

The action of the sacrificial cathode is seen in FIGS. 2a and 2b. As isillustrated in FIG. 2a, refractory hard metal current-conductingelements are disposed adjacent anodes 42 without any interveningsacrificial cathodes, a current field 44 is generated in the crust layerbetween the cathodes 40 and anodes 42. In FIG. 2b sacrificial cathodes46 are interposed between refractory hard metal cathodiccurrent-conducting elements 4t and anodes 42. The sacrificial cathodesonly extend into and through the rust layer in the electrolytic cell.rent passing between the anode and cathode through the crust layer willbe directed to the sacrificial cathode 46 and any sodium deposition orcorrosion occurring will take place on the sacrificial cathode and noton the refractory hard metal cathodic current-conducting element.

FIGS. 3, 4 and 5 illustrate three embodiments of the sacrificialcathode. As is seen the sacrificial cathode may take many forms, e.g. aspike, fork or spade. In FIG. 3 the sacrificial cathode is in the formof a spike. The sacrificial cathode may either be mechanically bolted toa cap member or welded thereto. In FIG. 3 the sacrificial cathode spike50 is depicted as being welded. FIG. 4 illustrates a fork arrangement ofsacrificial cathodes wherein the sacrificial cathode member 54 comprisesa plurality of spike members connected together and are shown attachedto a bolted cap member. FIG. 5 illustrates a spade configuration of thesacrificial cathode 58 with a bolted cap member 60.

The sacrificial cathode elements shown in FIGS. 3, 4

I and 5 need not be of large size and, consequently,-when used in anarrangement as shown in FIG. 1 strength is In such cases, small rods ofrefractory hard metal material such as titanium boride may be used forthe simple spikes.

Other possible cnostruction materials for sacrificial cathodes includecarbon, silicon carbide, SnO or other conducting metals, oxides, orceramics which possess suitable corrosion resistance combined with goodelectrical conductivity.

In FIG. 6 an embodiment is shown wherein the refratory hard metalcathodes are surrounded by a metallic frame which is connected to thecathode bus and acts as a sacrificial cathode for the refractory hardmetal elements. In FIG. 6 electrolytic cell 70 is shown having cell wall72 comprised of cell lining and insulation sections, and verticallydisposed anodes 74. The anodes depicted are of the pre-baked type,however, other anode forms such as Soderberg anodes may be employed. Asis seen, refractory hard metal cathodic current-conducting elements 76are supported by means 78 extending from the side wall of theelectrolytic cell 76 and extend toward the anodes which are centrallydisposed. Surrounding the anodes is a metallic \frame 84 which, as hasbeen described in FIG. 7, is electrically connected to the cathodicsystem. Both the refractory hard metal current-conducting elements andthe metallic sacrificial cathode frame are electrically connected toterminals 82 and to the cathodic bus 80.

The manner in which the sacrificial cathode metallic Thus, curtory hardmetal cathode through the crust layer.

frame 84 in FIG. 6 serves to protect the refractory hard metalcurrent-conducting element from corrosion in the crust layer of :thecell is described schematically in FIG. 7. As can be seen theelectrolytic cell 86 is shown with refractory brick contained within thefurnace shell 87. Disposed internally within the cell are refractoryhard metal current-conducting elements 88 which extend from the topthrough the crust layer and electrolyte into the moi-ten metal layer.Disposed between the refractory hard metal current-conducting element 88and anode 96 is the sacrificial cathode frame 92 which, as shown,extends only through the crust layer and serves as cathodic protectionfor current passing from the anode to the refrac- The sacrificialcathode assembly may, as shown, be secured to a side wall. Thesacrificial cathode assembly 90 is electrically connected to thecathodic bus system by electrical connection means 98. To avoidshorting, the sacrificial cathode assembly and the electrical connectioncan be insulated from the side wall by insulation 99. In the arrangementshown, as in the other sacrificial cathode arrangements within thepurview of the invention, the sacrificial cathode without interferingwith the current passage through the electrolyte and molten metal layerof the cell, prevents deposition on the cathode by current passingthrough the crust layer. The metallic sacrificial frame employed can bemade of any suitable metallic material which would not dissolvesignificantly at the temperatures employed in electrolytic celloperation and which will maintain its physical integrity under celloperating conditions. In addition, the sacrificial cathode, may beelectrically connected to an outside potential or other cathodic systeminstead of tothe same system to which the refractory hard metal cathodeis electrically connected.

Although the selection of materials used as sacrificial cathodes is, asdiscussed above, very wide, the choice of possible constructionmaterials must take into account the highly corrosive conditions in thecrust at 800 C. One material which has been found to be particularlysatisfactory ifO'I' large sacrificial cathode assemblies such as thefence in FIG. 6 is a high chromium cast iron which olfers a satisfactorycompromise between service life and replacement costs for thesacrificial cathode fence shown in FIG. 6. Moreover, sucha fence wouldhave the additional advantage of acting as a shock absorber to protectthe refractory hard metal cathodic current-conducting elements fromimpact breakage during anode changes.

The most complete protection against corrosion attack resulting fromelectrolytic conduction through the crust between the carbon anodes andthe refractory hard metal cathode elements is afforded by providing anelectrically conducting screen in the crust that completely eliminatesthe possibility of direct conducting paths between the anodes and therefractory hard metal elements. One advantageous form of such protectionis shown in FIGS. 6 and 7 described above, wherein a metallic frame ofsuitable electrically-conductive material is rigidly attached to thecell side Wall but electrically insulated therefrom. The sacrificialcathode frame extends into the crust region but does not penetratesignificantly into the liquid melt zone. By connecting the sacrificialcathode assembly to a cathode bus, a complete electrolytic screen forthe refractory hard metal elements is provided.

It is, of course, possible to modify this arrangement by using cathodicscreen elements arranged in the crust region between the carbon anodesand the refractory hard metal elements which are electrically connectedto the negative bus bar system but are not physically supported by thecell wall. Such electrodes might be made in several forms, all easilyremovable and replaceable. They should be arranged to reduce as far aspossible the electrolytic current passage between the anodes and therefractory hard metal elements but need not form a physically continuousscreen.

Still another embodiment of sacrificial cathodic protection for therefractory hard metal elements is described in FIGS. 8 and 9. As can beseen in FIG. 8, a cluster of refractory hard metal cathodes 100 arecentrally disposed within the cell. On each side of the cluster ofrefractory hard metal cathodic elements are disposed support members 112which extend across the cell and are supported by the side walls.Sacrificial cathode elements 108 are suspended from the hanging supportson both sides of the centrally disposed cluster of refractory hard metalelements. Conventional anodes arrangements may be employed on the remoteside of the sacrificial cathode assembly, permitting the elements 108 ofthe sacrificial cathode assembly to protect the centrally disposedcluster of refractory hard metal elements from corrosion by currentpassage through the crust layer between the anode and the refractoryhard metal cathodes.

The manner in which the anode, sacrificial cathodes, and refractory hardmetal elements of FIG. 8 are disposed within the cell with relation toeach other is further seen in FIG. 9, wherein the cluster of, refractoryhard metal elements represented by the single element 100 is shownpassing through the crust 102, electrolyte bath 104-, and the metal pad106 in the cell. Disposed on either side of the refractory hard metalelement are sacrificial cathode members 108'w hich, as is shown, extendthrough the crust layer 102 only. In this manner a current passagebetween anodes 110 and refractory hard metalelements through the crustlayer 102 is interrupted by intervening sacrificial cathode members 108.

The arrangement shown in FIGS. 8 and 9 represents an alternative methodof securing both mechanical and corrosion protection of top entryrefractory hard metal cathode elements utilizing a fewer number ofrelatively large sacrificial cathode elements. These elements may be inthe form of short, stubby, compact forms of refractory hard metalmaterial. These sacrificial elements can be suspended from a rigid steelsupport welded or clamped to the shell of the furnace of theelectrolytic cell. The short, -sacrificial elements will not be exposedto the molten aluminum and exposure to molten electrolyte will occuronly occasionally.

The arrangement shown in FIGS. 8 and 9 illustrates a technique wherein aminimum of cell space and electrolytic cell area would be used bydisposing the refractory hard metal cathode current-conducting elementscentrally in the cluster arrangement illustrated. The refractory hardmetal elements can be disposed in the cluster either at the ends of thecell or in a central compartment of the cell as shown. The anodes canhang on the opposite side of the cell with the sacrificial cathodemembers being interposed between the anodes and the refractory hardmetal cathodes. This arrangement, as indicated above, will offer inaddition to sacrificial cathode protection, significant protection frommechanical shock during breakage of the crust layer for anode changes.

It is apparent from the description that various changes andmodifications may be made without departing from the invention. Thescope of the invention should, therefore, be limited only by theappended claims, wherein What is claimed is:

1. In an electrolytic cell for the production of aluminum adapted tocontain a molten metal cathode layer, an electrolyte layer above saidmetal layer, a crust layer above said electrolyte layer, and having ananodic system comprising at least one anode adapted to extend throughsaid crust layer into said electrolyte layer and a cathodic systemcomprising at least one cathodic current conducting element adapted toextend through said crust and electrolyte layers into said metal layer,and wherein said element is comprised of refractory hard metal; theimprovement comprising sacrificial cathode means comprising electricallyconductive material interposed between said anode and refractory hardmetal conductor element, said sacrificial cathode means adapted toextend into said crust layer, means to electrically connect saidsacrificial cathode means to a cathodic system to enable current passagein the crust layer substantially between said anode and said sacrificialcathode means.

2. An improvement according to claim 1 wherein said sacrificial cathodemeans is electrically connected to the cathodic system of saidelectrolytic cell.

3. An improvement according to claim 1 wherein said sacrificial cathodemeans is electrically connected to a source of electrical potentialseparate from the cathodic system of the electrolytic cell.

4!. An improvement according to claim 1 wherein said sacrificial cathodemeans comprise refractory hard metal material.

5. In an electrolytic cell for the production of aluminum adapted tocontain a molten metal cathode layer, an electrolyte layer above saidmetal layer, a crust layer above said electrolyte layer, and having ananodic system comprising at least one anode adapted to extend throughsaid crust layer into said electrolyte layer and a cathodic systemcomprising at least one cathodic current conducting element adapted toextend through said crust and electrolyte layers into said metal layer,and wherein said element is comprised of refractory hard metal; theimprovement comprising a sacrificial cathode barrier comprisingelectrically conductive material interposed between said anode andrefractory hard metal conductor element, said sacrificial cathodebarrier adapted to extend into said crust layer, means to electricallyconnect said sacrificial cathode barrier to a cathodic system to enablecurrent passage in the crust layer substantially between said anode andsaid sacrificial cathode barrier.

6. An improvement according to claim 5 wherein said sacrificial cathodebarrier comprises a steel frame disposed transversely between the anodeand refractory hard metal cathodic current-conducting elements.

7. In an electrolytic cell for the production of aluminum adaptedto'contain a molten metal cathode layer, an electrolyte layer above saidmetal layer, a crust layer above said electrolyte layer, and having acathodic system comprising at least one centrally disposed refractoryhard metal cathodic current-conducting element adapted to extend throughsaid crust and electrolyte layers into said metal layer, and an anodicsystem comprising at least one anode disposed on either side-of saidcentrally disposed refractory hard metal cathodic element, theimprovement comprising sacrificial cathode means comprising electricallyconductive material interposed between said anodes and refractory hardmetal conductor element, said sacrificial cathode means adapted toextend into said crust layer, means to electrically connect saidsacrificial cathode means to a cathodic system to enable current passagein the crust layer substantially between said anodes and saidsacrificial cathode means.

8. An improvement according to claim 7 wherein said sacrificial cathodemeans comprise refractory hard metal material.

References Cited by the Examiner UNITED STATES PATENTS 476,914 6/1892Bernard 204-.-196 1,567,791 12/1925 Duhme 204231 3,028,324 4/ 1962Ransley 204--67 JOHN H. MACK, Primary Examiner.

H. S. WILLIAMS, Assistant Examiner.

1. IN AN ELECTROLYTIC CELL FOR THE PRODUCTION OF ALUMINUM ADAPTED TOCONTAIN A MOLTEN METAL CATHODE LAYER, AN ELECTROLYTE LAYER ABOVE SAIDMETAL LAYER, A CRUST LAYER ABOVE SAID ELECTRLYTE LAYER, AND HAVING ANANODIC SYSTEM COMPRISING AT LEASE ONE ANODE ADAPTED TO EXTEND THROUGHSAID CRUST LAYER INTO SAID ELECTRLYTE LAYER AND A CATHODIC SYSTEMCOMPRISING AT LEAST ONE CATHODIC CURRENT CONDUCTING ELEMENT ADAPTED TOEXTEND THROUGH SAID CRUST AND ELECTROLYTE LAYERS INTO SAID METAL LAYER,AND WHEREIN SAID ELEMENT IS COMPRISED OF REFRACTORY HARD METAL; THEIMPROVEMENT COMPRISING SACRIFICIAL CATHODE MEANS COMPRISING ELECTRICALLYCONDUCTIVE MATERIAL INTERPOSED BE-TWEEN SAID ANODE AND REFRACTORY HARDMETAL CONDUCTOR ELEMENT, SAID SACRIFICIAL CATHODE MEANS ADAPTED TOEXTEND INTO SAID CRUST LAYER, MEANS TO ELECTRICALLY CONNECT SAIDSACRIFICIAL CATHODE MEANS TO A CATHODIC SYSTEM TO ENABLE CURRENT PASSAGEIN THE CRUST LAYER SUBSTANTIALLY BETWEEN SAID ANODE AND SAID SACRIFICIALCATHODE MEANS.